A process cartridge of an image-forming apparatus including a cartridge frame, first and second plates assembled to both sides of the cartridge frame, and first through fourth guiding protuberances arranged on both sides of the cartridge frame, to guide attachment/detachment of the process cartridge to and from a cartridge mounting part formed on a main body frame of the image-forming apparatus. The first and the third guiding protuberances among the four guiding protuberances are formed on the first and the second plates, and the second and the fourth guiding protuberances are formed on both sides of the cartridge frame, respectively, and the first and the second through holes to receive the second and the fourth guiding protuberances in a passing through manner, are formed in the first and the second plates.